An evaporator that carries out heat exchange between exhaust gas of an internal combustion engine and water so as to heat the water by the heat of the exhaust gas and generate high temperature, high pressure steam is known from Japanese Patent Application Laid-open No. 2001-207839.
This evaporator improves the heat exchange efficiency by making the water flow in a direction opposite to the direction of flow of the exhaust gas, which is discharged from a combustion chamber of the internal combustion engine, that is, from the downstream side to the upstream side of the flow of exhaust gas, thus ensuring that there is a temperature difference between the exhaust gas and the water along the whole of a water passage.
However, in the above-mentioned conventional evaporator, since the flow of exhaust gas and the flow of water are opposite to each other, the temperature of water flowing through the water passage in the vicinity of the upstream end of an exhaust port (that is, the downstream end of the water passage) has already been increased, and it is difficult to cool the hottest section at the upstream end of the exhaust port sufficiently. Because of this, the heat of the high temperature exhaust port dissipates via the cylinder head, and the efficiency of recovery of the waste heat of the internal combustion engine is degraded, which is a problem. In order to avoid this problem, making the direction of flow of the exhaust gas and the direction of flow of the water parallel to each other enables the upstream end of the exhaust port, which reaches a high temperature, to be cooled effectively with low temperature water, but this results in an increase in the temperature of the water flowing through the downstream side of the exhaust port, where the temperature of the exhaust gas is low, and the temperature difference between the exhaust gas and water is therefore small in that section, thus degrading the heat exchange efficiency, which is a problem.